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Techtalk 126 – Supplement B: Cold Roof/Warm Roof Assemblies
Duradek’s recommendation is to defer to local building codes which are stipulated by local building inspectors. Alternatively, have a Design/Building Envelope Professional sign off on the assembly. Duradek is not the Design Professional and cannot provide any Design/Build Specifications.
Most all roof deck assemblies fall into one of 2 categories – Cold Roof Assemblies and Warm Roof Assemblies. Regardless of the category, Duradek Membranes make up the very top part of the assembly, as the Roofing Membrane and Walking Surface. As Duradek is only a part of the entire roofing assembly we can only be responsible for the waterproofing from above, and not for the design or performance of any other part of the assembly.
Cold Roof Assemblies (typical):
This assembly is the most common because there is no additional height added on top of deck causing door sill elevation issues. However, it’s more expensive and has issues with thermal inefficiencies (thermal bridging) – cold from the deck and joist travel into the joist cavity and into the room below. This thermal bridging causes heat loss and possible condensation concerns.
Definition: A cold deck roof is where the Insulation is below the deck (the plywood). Insulation such as butt insulation, rigid insulation, or SPF spray foam insulation is applied between the joist cavities. These assemblies are either vented (dead air space open to soffits), or non-vented (no dead air space) depending on climate zone and building code in local jurisdiction.
Potential Issues within a Cold Roof Assembly:
- Vented: Moisture can build-up in vented space, causing condensation on underside of plywood and dripping onto drywall ceiling. This will result in mold and mildew staining and ultimately wood rot.
- Non-vented: Plywood heaving (crowning up) between joists. The likely cause is trapped moisture in the plywood and joists. This will be evident when plywood under the membrane is wet, this will ultimately result in wood rot.
Considerations/Options to Discuss with the Design Professional:
- Moisture build-up can be mitigated or managed by either a vented or unvented roof. Either condition is stipulated by the local building code requirement, or recommendation of a building envelope professional.
Warm Roof Assemblies (typical):
This assembly is the most efficient because the insulation separates the deck (and space below) from the elements above and easy to install right on top of the deck surface. However, it is more expensive, adds substantial height to the overall assembly by leaving the joist space empty.
Definition: A warm roof/deck is where the Insulation is applied on top of the deck (the plywood). Rigid insulation (either tapered or flat) is typically Poly ISO or Poly Styrene. When being used as a walking surface a structural layer (2nd layer of plywood) needs to be applied on top of the rigid insulation.
Potential Issues within a Warm Roof Assembly:
- Moisture from rain or snow can accumulate within the assembly during the construction process. If moisture cannot, or has not, been removed from assembly then covering it with Duradek would trap the moisture causing mold, mildew, and rot.
- If the rigid insulation is not dense enough to support the butt edges of top layer of plywood (keeping the butt edges from moving independently) the vinyl will likely delaminate off the plywood and appear as a bubble at the butt joints. Vinyl could eventually split/crack at the butt joints.
- Door sill heights will likely have to be redesigned (raised) to accommodate the extra buildup caused by the tapered insulation. This complicates overall elevation designs of interior and exterior floors.
Considerations/Options to Discuss with the Design Professional
- Moisture accumulation can be mitigated or managed by either having drain holes in bottom layer of plywood or building to be horded to keep out moisture.
- Rigid insulation (tapered or flat) are rated for different loads. Heavy load-rated insulation may be able to support the butt edges of plywood.
- Plywood butt seam support can be achieved by placing metal coil stock under the plywood butt joints. By doing so would cause the butt joints to move monolithically.
Please consult a Design Professional/Building Envelope specialist if you note that one of the above conditions exists.
Within each of these assemblies, there are unique circumstances that could cause issues beyond our control once the Duradek (or any waterproofing/roofing membrane for that matter) has been applied. Issues include (but are not restricted to) trapped moisture in the wood components.